Optical fibers are being used widely in an ever increasing number of applications. Frequently, the fibers are subjected to a variety of stresses due to temperature and pressure changes, installation stress and the like. All of these can and do affect to one degree or another the fiber's characteristics and consequently, transmitted information validity. Strains have long been known to affect a fiber's lifetime since microcracks and catastrophic failure are more likely to occur where fibers are strained. Obviously, a means for determining such strain would be most helpful to the designer and technician to assure the long life and reliability of optical fibers.
Previous methods for measuring strain include directly measuring the change in time delay of an optical pulse transiting the fiber by means of a time delay generator and a sampling oscilloscope. This approach is dependent upon accuracy and stability of electronics and requires substantial capital equipment investments. Another approach uses electronic pulse regeneration circuitry to retransmit an optical signal, creating an electro-optical closed-loop oscillator. A consequence of this approach is that it is highly dependent on time delay variations in the electronic regeneration portion of the loop. Any changes in delay caused by strain in the fiber are compromised by the inherent instability and drift of the electronic components so that reliable indications of strain are difficult to obtain.
Thus, there is a continuing need in the state of the art for an optical fiber strain measuring device that relies on optical means for measuring longitudinal strain and avoids the time delay variations associated with electronic regeneration circuitry.